Federal agencies collect data on fatalities caused by “distracted driving,” which can include anything from talking on a cellphone to eating in the car. Although there have been laboratory and observational studies that have estimated the impact of cellphone usage while driving, there has been no way to tease out exactly how many crashes are caused by texting – or cellphone use overall.

“It’s extremely hard to measure causation,” says David Teater, senior director of the National Safety Council’s transportation strategic initiatives. “I believe [that] in all states there’s a mandatory blood alcohol test after a fatality.… There is nothing even close to that with distracted driving. There’s no way of knowing for sure.”

In the past, if the police did try to investigate whether cellphone usage was involved in a crash – not a regular occurrence, since for years talking on the phone while driving was not illegal – officers would typically have to rely on the driver’s word.

“There is going to be a lot of underreporting,” Mr. Wilson says. “In reality it’s probably an even worse problem than what the data is showing.”

Wilson and Stimpson combined data from the US Fatality Accident Reporting System database with cellphone usage statistics, adjusting for climate, state demographics, and other factors. Using this statistical modeling, they determined that if texting had not existed, the number of distracted driving fatalities would have actually declined from 4,611 to 1,925 per year from 2001 to 2007. As it is, distracted driving deaths increased to 5,870 in 2008, according to the US Department of Transportation.

Wilson and Stimpson also found a 19 percent increase in auto fatalities for every 1 million additional cellphone subscribers – a smaller, yet still significant, impact compared with texting.

“We basically concluded that cellphones are dangerous, there’s no question about that,” says Wilson. “But it’s really how people are using the cellphones.”

Their findings may help address one of the quandaries of distracted driving: Although the prevalence of texting and cellphone usage in cars has increased dramatically over recent years, the number of fatal car crashes in the US is actually down.

“Every study, and common sense, says that texting behind the wheel is a huge distraction because you’re taking your mind off the road but also your eyes off the road,” says Russ Rader, spokesman for the Insurance Institute of Highway Safety. “But we should be able to see something in the crash statistics reflecting this, and we’re not.”

Mr. Rader says his organization is researching this discrepancy.

According to the new study, those involved in distracted driving fatalities are more likely to be white, male, and under 30 than they were a decade ago. This is the same demographic most likely to be texting, the researchers point out.

Thirty states and Washington, D.C. now prohibit texting while driving; eight states plus D.C. outlaw any handheld cellphone use while driving, and Delaware will do so beginning next year. Enforcement, however, continues to be lax.